Wireless technologies require radio spectrum to operate. Radio spectrum is finite and highly regulated in all parts of the world. The increased usage of wireless devices is increasing the demand for additional spectrum to be used for wireless communications. As a result, radio spectrum is one of the most expensive resources for wireless communications operators.
New Long Term Evolution (LTE) technologies are being developed to make use of the unlicensed shared spectrum. The main goal for using LTE in unlicensed spectrum is to leverage the relatively large amount of unlicensed spectrum in order to provide a better mobile service. Some of these technologies are termed License Assisted LTE (LAA-LTE) or LTE Unlicensed (LTE-U). The initial focus for LAA-LTE is to use a small portion of the 5 GHz shared spectrum. In the USA and Canada, 250 MHz of spectrum is assigned for shared use in the 5 GHz bands.
Shared, or unlicensed spectrum, operates very differently compared to traditional licensed 3GPP spectrum. Users of unlicensed spectrum must share the spectrum resources with other users and other radio technologies, and do so in a fair way while at the same time ensuring that the spectrum is used as efficiently as possible.
Different technologies may use unlicensed spectrum simultaneously, e.g. By contrast, the LTE protocols and specifications have been developed on the premise that only LTE nodes operate in the same spectrum.
FIG. 1 illustrates a current cellular communication network configuration. LAA-LTE is used for illustration purpose. Within a cellular communication network, 5 GHz channels in the unlicensed spectrum are used as secondary component carriers 40 in a carrier aggregation configuration with a licensed LTE cell 30 (the Primary cell, PCell or Primary Carrier) for downlink (DL) transmission only to user devices 20. That is, the 5 GHz cells 40 are DL-only Secondary Cells (SCells) or Secondary Carriers. The uplink (UL) transmissions are sent using the PCell 30. It is noted that the notion DL-only secondary cells in the context of current LAA-LTE refers herein to the transmission of user data, but not all data, such as control data. It is noted that some control signals, as described in more detail below, are communicated from the SCells 40 to current LTE nodes. This aggregation of spectrum provides for a larger pipe with a more responsive user experience. By also maintaining a persistent anchor in the licensed spectrum to carry all of the control and signaling information, the user experience is more seamless and reliable.
One of the most significant changes being introduced for LAA-LTE is the requirement that an LAA-LTE base station, known as evolved Node B (eNB) periodically stops all transmissions in the unlicensed spectrum in order to allow other technologies to access the associated radio channel(s) and to perform measurements of other users of this (these) channel(s). This ON/OFF or discontinuous transmission (DTX) is in the order or tens to hundreds of milliseconds and may dynamically change based on demand and associated radio channel(s) usage.
In addition to the discontinuous transmission mechanism described above, there are many possible channels that may be utilized by an LAA-LTE eNB. For example, in the USA, the FCC has defined 3 5 GHz bands (termed the Unlicensed National Information Infrastructure bands, or UNII bands): UNII-1, UNII-2 and UNII-3/ISM (industrial, scientific and medical). Each of these bands covers several 20 MHz channels (4 in UNII-1, 16 in UNII-2 and 5 in the combined UNII-3/ISM band). These bands are illustrated in FIG. 2.
In order to communicate using SCells, SCell channel selection schemes are contemplated. There are several problems with the currently envisioned SCell channel selection schemes, leading to a need for improvements in cellular network communication using SCells.